By my calculations (who knows if that counter on the link to Apple's site is actually accurate or not) if it continues at the speed its at now, the billionth app download should happen somewhere between monnday night and tuesday morning (20th-21st) late at night.

Keep in mind that is in whatever time zone I'm in here in ATL, GA (I am drawing a blank at the moment )

Let me try to clear this up a little... "The prize will be awarded for the entry (either through an app download or through the non-purchase online entry) sent immediately following the download of the 999,999,999th app." This means the one billionth app downloader will be the one to win. It doesn't say, "...for the entry randomly picked after the download of the 999,999,999th app."

the clock is not a live download clock. i set my bios time to april 29th at 3 am and it hit a billion, if you dont believe me then try it yourself. so we will not know for sure when the billionth app is downloaded since the clock is not live, but my bets are on the 29th since thats when apple "predicts" the billionth app is downloaded.

two things are infinite: the universe and human stupidity; and i'm not sure about the the universe.

Or, everyone could just read the rules, where it clearly says that the billionth download (or form entry) wins the prize. Read the "Sweepstakes" section: Rules
It's exactly what they did when iTunes hit a billion, so they can do it again